A striped ocean hiding in plain sight

The ocean is constantly surprising us. A hard look at some ocean data …

It may seem hard to believe, but the oceans have stripes. These stripes are not visible without looking very closely, but they are visible through their effect on currents, temperatures, and sea surface heights. They were first spotted in a careful analysis of the Mean Dynamic Ocean Topography (MDOT) dataset, but had to be confirmed by looking directly at ocean buoy and vertical temperature profile data.

Global east-west ocean velocities (cm/s) Image: Maximenko et. al

These stripes are being reported in an upcoming issue of Geophysical Research Letters by an international group of scientists lead by Nikolai Maximenko at the University of Hawaii. Each stripe is approximately 200km across and they are oriented roughly east-west. In the northern hemisphere, they appear to be oriented more southwest-northeast at an angle of 13 degrees, while in the southern hemisphere they are closer to northwest-southeast at an angle of 9 degrees.

Finding these stripes was difficult because they are very subtle features that are superimposed on much larger ocean current, topography, and temperature fields. The stripes have velocities around 1-1.5cm/s, while major ocean currents often travel at 40-50cm/s. The change in sea surface height from one stripe to the next is roughly four centimeters—globally, the average sea surface height varies by one to two meters. Likewise, the temperature at a depth of 100 meters varied by 12 degrees Celsius in one study area, while the variation across stripes was approximately one degree.

So how did they find these stripes? Essentially, they just filtered out any larger spatial scale features. Most changes in sea surface height, velocity, and temperature occur over thousands of kilometers. By using two consecutive high-pass filters, the team was able to remove the larger scale features.

These stripes are interesting for a number of other reasons. For one thing, they persist to at least 700 meters deep based on the temperature profile data. In another interesting twist, the velocity within the stripes does not always follow the orientation of the stripes.

Finally, while the exact cause of these features is unknown at present, they are present in the ocean general circulation model run on the Earth Simulator supercomputer in Japan. By running the same high pass filter over the model output they were able to isolate the same features they saw in the ocean data.

There were some differences with observations, as the model data in the northern hemisphere had a wider spacing (450km wavelength vs 400km), and the model stripes were only tilted five degrees. In the southern hemisphere, the model showed a closer match to the data with a modeled wavelength of 375km and a tilt of seven degrees (compared to 400km and nine degrees in the data). Finally, while we do not have measured data deep enough to verify this, the model suggests that these features are coherent all the way to the sea floor.

The precise cause of these stripes remains a mystery, although the authors have a few thoughts on the matter. They suggest that the stripes may be caused by a form of inertial waves known as Rossby waves that are driven by the coriolis force. They may be a more general phenomenon, too, as others have compared these stripes to the cloud bands that are observed in the atmosphere of Jupiter.